1. Field of the Invention
The present invention relates to a surface wave motor for driving a rotor on a ring-shaped vibration member by a travelling surface wave generated in the vibration member.
2. Description of the Prior Art
The surface wave motor translates a vibration motion created when a periodic voltage is applied to an electrostrictive element to a rotational motion or linear motion. Since it requires no winding as compared to a conventional electromagnetic motor, it is simple in structure, of small size and produces a high torque even at a low rotating speed. Accordingly, interest therein has been recently increasing.
FIGS. 1 and 2 illustrate the principle of drive of a prior art surface wave motor. FIG. 1 illustrates generation of a surface wave in the surface wave motor. Electrostrictive elements 2a and 2b bonded to a metal vibration member 1 are arranged on upper and lower sides of the vibration member 1 for the sake of convenience of explanation, but they are actually arranged on one side of the vibration member 1 in the spaced relation to meet a requirement of spatial phase difference of .lambda./4.
The vibration member 1 is used as one electrode for the electrostrictive elements 2a and 2b, and an A.C. voltage of V=V.sub.0 sin .omega.t from an A.C. power supply 3a is applied to the electrostrictive element 2a while an A.C. voltage of V=V.sub.0 sin (.omega.t.+-..pi./2) having a phase difference of .lambda./4 is applied to the electrostrictive element 2b through a 90.degree. phase shifter 3b. Signs (+) and (-) in the above formula are selected by the phase shifter 3b depending on a direction of movement of a movable member 5. Let us assume that the sign (-) is selected so that the voltage of V=V.sub.0 sin (.omega.t-.pi./2) is applied to the electrostrictive element 3b. If only the electrostrictive element 2a is vibrated by the voltage of V=V.sub.0 sin .omega.t, a vibration by a standing wave is generated as shown in FIG. 1(a). If only the electrostrictive element 2b is vibrated by the voltage of V=V.sub.0 sin (.omega.t-.pi./2), a vibration by a standing wave is generated as shown in FIG. 1(b). When the two A.C. voltages having the phase difference are simultaneously applied to the respective electrostrictive element and 2a and 2b the surface wave travels. FIG. 1(A) shows a wave at t=2n.pi./.omega., FIG. 1(B) shows a wave at t=.pi./2.omega.+2n.pi./.omega., FIG. 1(C) shows a wave at t=.pi./.omega.+2n.pi./.omega. and FIG. 1(D) shows a wave at t=3.pi./2.omega.+2n.pi..omega.. A wavefront of the surface wave travels in an X direction.
Such a travelling wave includes a longitudinal wave and a lateral wave. Looking at an apex A of the vibration member 1 shown in FIG. 2, the movable member 5 is press-contacted to the surface of the vibration member 1 which makes a counterclockwise rotating elliptic motion with a longitudinal amplitude u and a lateral amplitude w and it contacts only the apex of the vibration plane. Accordingly, the movable member 1 is driven by a component of the longitudinal amplitude u of the elliptic motion of mass points A, A', . . . at the apex and moved in a direction of arrow N.
When the phase is shifted by +90.degree. by the 90.degree. phase shifter, the vibration wave travels in a -X direction and the movable member 1 is moved opposite to the direction N.
A velocity at the apex of the mass point A is V=2.pi..function.u (where .function. is a vibration frequency) and a velocity of the movable member 5 depends thereon and also depends on the lateral amplitude w because of a friction drive by the press-contact.
Thus, the velocity of the movable member 5 is proportional to the magnitude of the elliptic motion of the mass point A, and the magnitude of the elliptic motion is proportional to the voltage applied to the electrostrictive elements.
Accordingly, a high voltage is required to obtain a high rotating speed, and a high drive efficiency is hardly attained.
It is difficult to support the vibration member 1 without impeding the vibration. It is usually supported by felt material. However, precise positioning is difficult to attain and the material changes by aging. Accordingly, many problems are included in packaging the motor.
U.S. Pat. No. 4,104,553 discloses a technique to support a piezoelectric vibrator such as a crystal vibrator by a thin metal wire of an appropriate length determined by a predetermined condition at a node of the vibration or the smallest amplitude point so that the vibration of the vibrator is not propagated externally and an external disturbance is also not propagated to the vibrator. However, this technique relates to the technique to support a rod-like vibrator and it does not teach an appropriate method to support a ring-shaped vibrator.